The school resource officer forced to resign after he failed to engage the shooter during the Feb. 14 massacre at Marjory Stoneman Douglas High School in Parkland, Florida, has broken his silence.

Scot Peterson, 55, told The Washington Post that coping with the mass shooting, which left 17 people dead and 17 others injured, and his failure to mitigate the attack has been “haunting” him.

“I’ve cut that day up a thousand ways with a million different what-if scenarios, but the bottom line is I was there to protect, and I lost 17,” Peterson told The Post.

Peterson, a former Broward County sheriff’s deputy, has faced an intense backlash in the wake of the massacre. He resigned as Stoneman Douglas high school’s armed resource officer in late February after videos showed he did not enter the school during the shooting.

In April, the father of one of the shooting victims filed a wrongful death lawsuit against Peterson, a 33-year law enforcement veteran.

“I just want to expose what a coward [Peterson] was and that he could’ve saved everybody on the third floor,” Andrew Pollack, whose 18-year-old daughter Meadow was killed, told the Sun Sentinel. “I don’t want him to go anywhere in the country and not have people recognize what a coward he is.”

Peterson has defended his conduct during the shooting, steadfastly denying that he did “nothing” as gunshots rang out in the school and noting that he hadn’t been sure at the time whether the attack was happening inside or outside of the building.

“I’m getting on the radio to call in the shooting,” Peterson told The Post, reliving his actions on that tragic day. “I’m locking down the school. I’m clearing kids out of the courtyard. They have the video and the call logs. The evidence is sitting right there.”

“There wasn’t even time to think,” he added. “It just happened, and I started reacting.”

Peterson’s first TV interview since the attack is set to air Tuesday on NBC’s “Today” show.

“Knowing what I know today, I would have been in that building in a heartbeat,” Peterson can be heard saying in a preview of the interview. “It was my kids. It’s just ― I didn’t know.”